Acute myeloid leukemia (AML) is a devastating disease affecting 10,000 people per year in the U.S. Most patients with AML either are refractory initially or become resistant to chemotherapy within one to two years. Salvage allogeneic bone marrow transplantation is currently available only for a minority of patients and is associated with significant morbidity, relapses and mortality. Novel therapies which can overcome drug resistance with tolerable toxicities are sorely needed. We have genetically engineered a diphtheria fusion toxin, DTGM, consisting of the catalytic and translocation domains of diphtheria toxin (DT) fused to human granulocyte-macrophage colony-stimulating factor (GM-CSF). In an ongoing phase I clinical trial with 20 patients treated to date, biological activity has been observed at low doses associated with minimal drug related toxicities. Patients with minimal disease burden (low or absent circulating blasts, <50 percent marrow replacement and no splenomegaly) responded best. No evidence of drug-related myelosuppression or neurotoxicity was observed. In previous tissue culture experiments, dramatic synergy was observed between DTGM and cytarabine. Based on this clinical and preclinical evidence, we propose a pilot phase I/II clinical trial of DTGM combined with continuous infusion cytarabine in patients with relapsed or refractory AML. In Specific Aim 1, dose-limiting toxicities will be defined and damage to normal hematopoietic progenitors (CFU-GM, CFU-GEMM and LTC-IC) determined. In Specific Aim 2, the response rate of the combination will be estimated and compared to the historical response rates for similar patients treated with DTGM or cytarabine alone. Patient leukemic progenitor sensitivity (AML-CFC) to the combination regimen will be correlated with response rate. Specific Aim 3 will evaluate patient leukemic blast mRNA expression patterns and identify genes influencing the response rate or toxicity profile. We anticipate these studies should provide the basis for the design and implementation of pivotal multi-institutional phase II and III trials of DTGM/cytarabine therapy for relapsed/refractory AML.